Automated analyzers are used in clinical laboratories to measure various chemical constituents of body fluids, such as whole blood, blood serum, blood plasma, cerebral spinal fluid, urine, and the like obtained from patients. Automated analyzers reduce the number of trained technicians required to perform the analyses in a clinical laboratory, improve the accuracy of the testing, and reduce the cost per test.
Typically, an automated analyzer includes an automated fluid moving system which aspirates a sample of body fluid from a patient's specimen container and dispenses the sample into a reaction cuvette. The fluid moving system typically includes a pipette or sample probe on a robotically controlled arm to perform the aspiration and dispensing functions.
Chemical reagents, which are specific to the test being performed, are disposed into the sample-containing cuvette, thereby mixing the sample with the chemical reagents. By examining the reaction products resulting from the mixing of the sample and reagents, the automated analyzer determines the concentration of the specific chemical constituent being tested. Upon completion of the test, the automated analyzer typically prints the results of the test, including a sample identifier, a numerical result of the test, and a range of values for the chemical constituent as measured by the test.
During an aspiration operation, the robotic arm, under the command of a system controller, positions the sample probe above a specimen container and moves the probe into the container until the probe reaches the fluid in the container. A syringe type pump is activated to draw sample fluid from the specimen container into the probe. To ensure that accurate results are obtained in the tests, a consistent known volume of the sample must be accurately aspirated and delivered to the reaction cuvette. Under ideal conditions, motorized syringes can deliver the volume at the needed accuracy. However, conditions are not always ideal, so a method of verifying sample volume is needed.
Prior art methods have focused on detecting non-ideal conditions. In one method, pressure is measured after each increment of aspiration. A pressure value outside a predetermined pressure range signals heterogeneity in the sample. Khalil, Omar S. et al., "Abbott Prism: A Multichannel Heterogeneous Chemiluminescence Immunoassay Analyzer," Clin. Chem., 37/9, 1540-47 (1991). European Patent Application No. 341,438 describes a system in which pressure is also monitored during aspiration. Bubbles, a clot, or a pressure leak are shown on a display screen as one or more spikes. European Patent Application No. 215,534 describes a system in which pressure after a suction operation is measured and compared to an expected normal value.